Definitions vary from the strangely tautological ("An expert system is a
computer application that functions like an expert," says one online encyclopedia)
to the exceptionally technical.

Expert systems, which are considered part of the field of artificial
intelligence, generally attempt to capture the decison-making processes
of human experts and represent them in the form of inferential "if . .
. then" statements along the lines of "If the light is green, then step
on the gas."

Cerner’s Vanetta Wick offers this more detailed definition: "Our rule-based
expert knowledge system allows, through the use of English-language statements
in the case of Millennium [Cerner’s latest version of its suite of PC-based
products], what we call parameters, entry points that allow users to specify
their particular interest level or their particular database entry. These
statements, put together, make up what we call a rule, an expert knowledge
module that runs on the customer’s system. "The statements are templates
that drive system functionality," adds Wick. "There are evoke templates,
which start the rule, logic templates, which are what the software evaluates,
and action templates, which represent what it’s going to do based on what
it found."